Various techniques for ensuring the safety of a driver by mounting an airbag apparatus on a vehicle such as a motorcycle are conventionally known. For example, a technology is known in which an airbag housed in a case attached to a vehicle body frame is unfolded and inflated by an expanding gas upon a front collision of an automotive two-wheeler, thereby restraining the driver (for example, see Japanese Unexamined Patent Application Publication No. 2002-137777). This technology presents the possibility, in terms of a motorcycle, of ensuring a large protecting area provided by the airbag by connecting walls spaced apart from each other forward and backward of the airbag within the airbag by means of a tether. Furthermore, there is an increasing demand to improve driver protection by developing efficient driver-restraint configurations by means of airbags deployed upon the occurrence of an accident.